In measurement systems such as digital oscilloscopes, consecutively sampled values of an applied waveform are digitized, stored in a memory, and then used to reconstruct the waveform as a displayable image (the “trace”) on a display device by reading and processing the stored values. The stored digital values are typically referred to as an acquisition record, the contents of which correspond to a definite time interval in the history of the applied waveform. The length of the time interval is largely determined by the number of addressable memory locations dedicated to signal acquisition and the rate at which the samples are acquired.
Many activities that are performed with an oscilloscope require that the displayed portion of the acquisition record be in some defined relationship to a detected event in the waveform, such as a rising or falling edge of the waveform, for example. The detected event is commonly referred to as a trigger event, or trigger. When the trigger event being detected is a condition of the waveform itself, the event is referred to as an internal trigger event. When the trigger event being detected is a condition outside of the waveform that has some relationship to the waveform being measured, such as another waveform, the event is referred to as an external trigger event. In response to a detected trigger, some subset of the acquisition record is typically displayed to allow panning and zooming of the trace.
FIG. 1 illustrates a time-versus-voltage plot of a portion of a waveform 2 that includes a rising edge 3 and a falling edge 4, which will be used to define what is meant by an edge trigger, as that term is used herein. A trigger threshold voltage level 5 is the voltage level at which the oscilloscope should trigger. For the rising edge 3, the lower threshold voltage level 6 defines the hysteresis band. To cause a trigger on the rising edge 3, the signal must be below the lower threshold voltage level 6 and then cross above the trigger threshold voltage level 5. For the falling edge 4, the upper threshold voltage level 7 defines the hysteresis band. For the falling edge 4 to cause a trigger, the signal must cross from above the upper threshold voltage level 7 to below the trigger threshold voltage level 5. For a standard edge trigger, there is no time limit for how long it takes for the signal to cross between the lower or upper threshold voltage levels 6 and 7, respectively, and the trigger threshold voltage level 5, although special triggering modes can have such time limits.
Until recently, oscilloscope triggering was performed by analog circuitry running at the specified trigger bandwidth. Although a digital trigger circuit has been developed, most oscilloscope trigger circuits are still analog circuits (i.e., continuous time). Rather than operating on the analog signal directly, a digital trigger circuit operates on the data after it has been digitized by an analog-to-digital converter (ADC), and therefore operates in the discrete time domain.
The higher the bandwidth of the oscilloscope, the more difficult it is to develop the analog trigger circuit. The analog trigger circuit includes a comparator, which is essentially a very high gain amplifier. A high bandwidth comparator pushes the Gain-Bandwidth product of available technologies, so trigger circuit bandwidths are usually much lower than the signal bandwidth for the highest-bandwidth oscilloscopes. For example, an oscilloscope with a signal bandwidth that is greater than 60 Gigahertz (GHz) may have a trigger circuit BW of only 20 GHz.
The highest-bandwidth digital oscilloscopes time interleave multiple ADCs to achieve the required sample rates. With existing digital trigger circuits, all of the digital comparison results for the trigger channel from determining whether the signal is above the higher threshold level, below the lower threshold level, or in between the lower and higher threshold levels must be brought together in one place and then processed at the full sample rate of the oscilloscope. With very high sample rates, e.g., sample rates greater than 100 Gigasamples per second (GSa/s), this presents difficulties, particularly in terms of signal routing and power consumption.
A need exists for a high-bandwidth measurement system having a digital edge trigger circuit that is capable of operating at the full signal bandwidth of the measurement system and that avoids the aforementioned difficulties in terms of signal routing and power consumption.